1. Field of the Invention
This invention relates to communication systems, and more particularly to methods for frame matching in wireless communication systems.
2. Description of Related Art
Wireless communication systems facilitate two-way communication between a plurality of subscriber mobile radio stations or “mobile stations” and a fixed network infrastructure. Typically, the plurality of mobile stations communicates with the fixed network infrastructure via a plurality of fixed base stations. Exemplary systems include such mobile cellular telephone systems as Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, and Frequency Division Multiple Access (FDMA) systems. The objective of these digital communication systems is to provide communication channels on demand between the mobile stations and the base stations in order to facilitate communication between the mobile station users, and also to allow connectively with a fixed network infrastructure (e.g., a wired “POTS” telecommunications system). An exemplary digital communication system is shown in FIG. 2 and described in detail as FIG. 1 in the above-incorporated U.S. application Ser. No. 09/687,700 (now U.S. Pat. No. 6,891,853).
Another exemplary digital communication system is now described with reference to FIG. 1. As described hereinbelow and in the incorporated U.S. application Ser. No. 09/687,700, digital communication systems transmit data between wireless devices over an airlink. As shown in FIG. 1, a transmitting device 10 communicates with a receiving device 20 via an airlink 5. The airlink 5 comprises a forward link 2 and a reverse link 4. The transmitting device 10 comprises a coder 14 operatively connected to a radio 12. The receiving device 20 comprises a radio 22 operatively connected to a decoder 24.
As shown in FIG. 1, the transmitting device 10 processes data and transmits symbols across the airlink 5. The coder 14 inputs data streams that are generated by devices within the transmitting device 10 (such as a well-known vocoder, not shown in FIG. 1). The coder 14 processes the data and outputs encoded symbols, or “data groups”, at a predetermined symbol code rate. Symbol code rates are defined as the number of symbols that an encoder generates per frame. Symbol code rates are sometimes referred to as native symbol rates. These symbols are configured into well-known “frame structures” or “frames”. Exemplary frames and frame structures are described in chapters 2 and 3 of the TIA specification entitled “Physical Layer Standard for cdma2000.2 Standards for Spread Spectrum Systems”, TIA/EIA/IS-2000.2-A, published in March 2000 by the Telecommunications Industry Association (TIA), and is hereby incorporated by reference for its teachings on CDMA communication systems, and is referred to hereafter as “IS-2000.2”.
Referring to FIG. 1 and as is well known, radio 12 transmits frames of data across the airlink 5 (i.e., at the physical layer) to the receiving device 20 at a specified transmission code rate. Transmission code rates are defined by the number of symbols transmitted per frame. Transmission code rates are sometimes referred to as symbol rates of a particular frame configuration. The decoder 24 receives the frames and outputs data for use by other components in the receiving device 20. Typically, digital communication systems, such as CDMA systems, utilize a plurality of multi-sized frame structures.
Exemplary prior art CDMA systems are described in the TIA specification, entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” TIA/EIA/IS-95-A, published in May 1995 by the Telecommunications Industry Association, and referred to hereafter as “IS-95A”. The update and revision to IS-95A and J-STD-008 (PCS specification analogous to IS-95A) is TIA/EIA/IS-95-B, which was published in March 1999 by the Telecommunications Industry Association (TIA), and is referred to hereafter as “IS-95B”. The IS-95A and IS-95B specifications jointly specify the second generation, or “2G”, CDMA system. More recently, a third generation, or “3G” CDMA system, is described in the TIA specification, and is entitled “cdma2000 Series”, TIA/EIA/IS-2000-A. The TIA/EIA/IS-2000-A specification was published in March 2000 by the TIA, and is referred to hereafter as “IS-2000”. The IS-95A, IS-95B and IS-2000 specifications are hereby incorporated by reference for their teachings on CDMA communication systems. In 2G systems, only a limited number of usable data frame sizes are utilized because data must be encoded at a specified symbol code rate. In contrast, 3G systems support arbitrary data frame sizes.
In order to support arbitrary data frame sizes, one encoding method, referred to as a “data-to-frame matching” method, attempts to overcome this drawback by “matching” (or aligning) the specified symbol code rate to the specified transmission code rate. In accordance with this encoding method, encoded symbols are “fitted”, or adjusted to fit into, standard frame structures using techniques such as symbol repetition and puncturing. Symbol or code repetition is defined as the concatenation or addition of redundant code symbols from a sequence that is generated by an encoder.
Consequently, a lower rate coder can be used, or equivalently, the amount of symbols that are transmitted can be increased. Symbol or code puncturing is defined as the deletion of code symbols from a sequence generated by an encoder. A higher rate coder results, or equivalently the amount of symbols that are transmitted is reduced.
For example, in a CDMA system configured in accordance with IS-2000, encoded symbols having a symbol code rate of L symbols per frame must be matched with a transmission scheme having a transmission code rate (or processing rate) of N symbols per frame, where N is greater than L. Accordingly, for any given frame, the L symbols are repeated M times, where M is the smallest integer such that (M*L)>N, where “*” is defined as the well-known mathematical multiplication function. The (M*L) symbols are then reduced to N symbols by puncturing (i.e., deleting) P symbols, where P=((M*L)−N). When symbols are properly punctured (i.e., punctured without error) no appreciable loss of information occurs because, as is well known in CDMA systems, many symbols are redundant to provide robust protection against transmission errors. Thus, the proper deletion of a few redundant symbols typically does not cause significant problems, especially when the redundant symbols are spaced far apart from each other. Symbol repetition and puncturing techniques, basic radio system parameters and call processing procedures for prior art CDMA systems are described in the above-incorporated IS-95A, IS-95B and IS-2000.
Accordingly, one objective of digital communication systems is to match data having flexible data lengths to multi-sized frames, and thus facilitating the transmission of data on the physical layer. One exemplary data-to-frame matching method and apparatus that attempts to achieve this objective is now described with reference to FIG. 1. Referring again to the exemplary digital communication system 100 of FIG. 1, the coder 14 includes an encoder that is operatively connected to a rate matching circuit (not shown in FIG. 1). The exemplary encoder received raw data and outputs encoded symbols. For example, as described in IS-95A with reference to FIGS. 6.1.3.1–2 at pages 6–8, an encoder may perform the following functions: include frame quality indicators, add an 8-bit encoder tail and perform convolutional encoding. The encoder outputs the encoded symbols to an input of the rate matching circuit.
The exemplary rate matching circuit matches the input symbols to multi-sized frames in accordance with the symbol code rate and the transmission code rate of the frame. In accordance with the exemplary data-to-frame matching method, the rate matching circuit is capable of performing three functions: 1) standard rate matching via either simple repetition or periodic deletion of symbols; 2) flexible symbol repetition (utilized in cases where the symbol code rate is less than the transmission code rate that is associated with the specified frame and simple repetition or periodic deletion of symbols does not produce a required transmission rate.); and 3) flexible symbol puncturing (utilized in cases where the symbol code rate is greater than the transmission code rate that is associated with the specified frame and simple repetition or periodic deletion of symbols does not produce a required transmission rate.) Thus, the rate matching circuit of the exemplary method performs standard rate matching, symbol repetition or puncturing to match the symbols to the specified frames.
One disadvantage of the above-described exemplary data-to-frame matching method is that utilization of non-standard puncturing results in unpredictable performance. Performance is defined as how well a method functions. In communication systems, performance is typically measured by using the signal-to-noise ratios needed to achieve a certain pre-specified information bit error rate (BER). In the above-described exemplary data-to-frame matching method, the performance of the system is unpredictable. Under certain scenarios, the performance improves, however, in other scenarios, it worsens. Most CDMA systems, such as those defined by IS-2000, typically require that performance levels remain above predefined minimum levels. Performance levels can drop below minimum levels due to the deleterious effects that are associated with non-standard puncturing. Another disadvantage of the exemplary data-to-frame matching method results from the utilization of non-standard puncturing, which results in increased design, increased manufacturing costs and complexity.
Therefore, a need exists for a frame matching method and apparatus which augments existing, or standard, data puncturing techniques in communication systems. The frame matching method and apparatus should match data streams having a plurality of different code rates to various frame sizes. Such a frame matching method and apparatus should maintain a minimum level of performance. In addition, the frame matching method should utilize existing or standard data puncturing techniques without adding undue costs and complexities. The present invention provides such a frame matching method and apparatus.